In recent years, spread of portable terminals, such as cellular phones and PDAs (Personal Digital Assistants; portable information terminals), have been remarkable. One factor of the rapid spread of these portable terminals is, for example, liquid crystal displays included as a screen display part of the portable terminals. The reason for this is that the liquid crystal displays are display devices that have a characteristic of requiring no power for driving themselves in principle, and therefore are of low power consumption.
The portable terminal employs as its power supply a battery of a single supply voltage. In contrast, in the liquid crystal display, a horizontal drive circuit for driving pixels arranged in rows and columns employs DC voltages that are different between a logic part and an analog part thereof. Furthermore, a vertical drive circuit for writing information to the pixels employs a DC voltage having the absolute value larger than that of the voltages for the horizontal drive circuit. Therefore, used for the liquid crystal display included in the portable terminal is a supply voltage conversion circuit that converts a single DC voltage to plural kinds of DC voltages having different magnitudes, a so-called DC-DC converter.
Conventionally, DC-DC converters employing an inductor are typically used for liquid crystal displays. However, due to recent trends of portable terminals toward lower power consumption and smaller size, use of charge pump DC-DC converters has been increasing (for example, refer to Japanese Patent Laid-open No. 2002-176764 (in particular, paragraphs 0005 to 0013 and FIGS. 11 to 14)). The charge pump DC-DC converter has an advantage of contributing to miniaturization of the portable terminal since it eliminates a need to use an inductor as an external part, although the current capacity thereof is comparatively small.
In a conventional charge pump DC-DC converter, the gate voltage of a transistor for driving a flying capacitor is controlled by changing its voltage swing controlled by the input voltage. Therefore, when realization of a DC-DC converter of large current capacitance is aimed, large size transistors need to be formed, which leads to a large circuit size. This circuit size increase is problematically significant when the circuits are fabricated using devices having comparatively small mobility such as low-temperature poly-silicon TFT (Thin Film Transistor), which are formed on an insulating substrate. Specifically, when forming a charge pump DC-DC converter on a so-called frame (peripheral region of a display area part) of a display integrally with the display area part (pixel part), the increase of circuit size of the DC-DC converter causes the increase of the frame size. Therefore, a serious problem arises when miniaturization of a display is aimed.
The present invention is made in consideration of the above-described problem, and a desire thereof is to provide a supply voltage conversion circuit allowing fabrication of a charge pump circuit having large current capability with a small area, a control method thereof, a display including the supply voltage conversion circuit, and a portable terminal employing the display as its screen display part.